1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and, more particularly, to a vertically aligned (VA) liquid crystal display in which liquid crystal molecules are aligned substantially vertically to the substrate when no voltage is applied to the liquid crystals and in which a phase difference film is used as a viewing angle compensation film to reduce viewing angle dependency that is characteristic of liquid crystal displays.
2. Description of the Related Art
Liquid crystal displays, more particularly, TN (twisted nematic) display mode TFT (thin film transistor) LCDs utilizing the rotary mode have recently spread as displays for personal computers and the like. However, such TN LCDs have problems such as a reduction of display contrast in a diagonal viewing direction, inversion of brightness and darkness of display and an insufficient response speed. Under such circumstances, VA LCDs have been proposed which utilize the birefringent mode instead of the rotary mode unlike TN LCDs.
VA LCDs are characterized in that they are higher than TN LCDs in display contrast and in the speed of response in switching between white and black levels (from white display to black display or from black display to white display). However, VA LCDs have the problem of viewing angle dependency during display of halftones similarly to TN LCDs.
So-called alignment separating techniques have been proposed and put in use to achieve preferable viewing angle characteristics by aligning liquid crystal molecules in a pixel in two or more different directions. In Japanese patent application No. H10-185836, the present applicant has proposed a so-called MVA (multi-domain vertical alignment) liquid crystal display having domain defining structure in which liquid crystal molecules in a pixel are aligned in two or more different directions by providing domain defining structure that is constituted by protrusions or recesses provided on opposite surfaces of a pair of substrates of the liquid crystal panel or slits provided on the electrode or a combination of such features.
FIGS. 1 through 3B are schematic illustrations of a conventional MVA LCD. FIG. 1 shows a general configuration of the same; FIG. 2 shows the region of one pixel; and FIGS. 3A and 3B are sectional views of a major part showing an operation of the same.
Referring to FIG. 1, a pair of polarizers 22 and 21 are provided outside a liquid crystal panel 30, and the absorption axes of the pair of polarizers 22 and 21 are orthogonal to each other as indicated by the arrow in the figure. Liquid crystal molecules are aligned in four directions in each pixel, and the directions of alignment of the liquid crystal molecules indicated by the arrows A through D are at an angle of about 45 deg. to the absorption axes of the polarizers 22 and 21.
Referring to FIG. 2, a plurality of gate bus lines 11 and drain bus lines 12 are disposed on a TFT substrate such that they cross each other, and a region surrounded by gate bus lines 11 and drain bus lines 12 serves as one pixel region. In a pixel regions, a TFT 306 is disposed which is connected to a gate bus line 11 and drain bus line 12, and a pixel electrodes 13 is provided in connection to the TFT 306. At the pixel electrode 13, a slit 15 to serve as domain defining structure is formed in a substantially V-shaped configuration, and it extends in directions substantially at an angle of 45 deg. to the polarizer axes of the pair of polarizers indicated by cross arrows. A storage capacity bus line 16 is disposed in the middle of the pixel in parallel with the gate bus line 11 to form a storage capacity (auxiliary capacity) between the pixel electrode 13 and itself. Protrusions 14 to serve as domain defining structure are disposed on a counter substrate. The protrusions 14 are formed in parallel with the slit 15 such that they do not overlap the same and are formed in a substantially V-shaped configuration similarly to the slit 15.
FIG. 3A shows a state at a time when no voltage is applied, and FIG. 3B shows a state at a time when a voltage is applied. Those figures show only elements that must be shown for description and omit other elements. A slit 15 is provided on a pixel electrode 13 disposed on a transparent substrate 24 that is a TFT substrate made of glass or the like, and protrusions 14 made of a dielectric material are provided on a counter electrode (common electrode) 25 provided on a transparent substrate 23 that is a counter substrate. Nematic liquid crystals having negative dielectrically isotropic properties are sealed between the TFT substrate and counter substrate. A pair of polarizers 22 and 21 is provided outside the transparent substrates 24 and 23.
When no voltage is applied, liquid crystal molecules 27 are aligned such that their longitudinal directions are substantially perpendicular to the substrates 24 and 23 as shown in FIG. 3A. When a voltage is applied, electric lines of force 28 are tilted by the slit 15 (or the cut-out in the pixel electrode 13) and the protrusions 14 as shown in FIG. 3B. Since the liquid crystal molecules 27 are tilted perpendicularly to the electric lines of force 28, they are tilted in two directions (i.e., directions 180 deg. different from each other) that extend from the slit 15 and the protrusions 14. Since the slit 15 and protrusions 14 in the pixel extend in directions about 90 deg. different from each other as shown in FIG. 2, four regions having different directions of alignment A through D are formed as shown in FIG. 2. The directions of alignment A through D correspond to the reference symbols A through D in FIG. 1.
FIG. 4 is a diagram showing viewing angle characteristics of the conventional MVA LCD shown in FIGS. 1 through 3B.
FIG. 4 shows that the MVA LCD has contrast of 100 or more even it is viewed in positions at a tilt angle of 80 deg. in upward, downward, leftward and rightward directions (at azimuth angles of 0, 90, 180 and 270 deg.) and that it therefore has preferable viewing angle characteristics in the upward, downward, leftward and rightward directions.
However, a significant reduction occurs in the contrast when viewed in diagonal directions at an angle of 45 deg. (at azimuth angles of 45, 135, 225 and 315 deg.). Contrast of 5 or more can be achieved in a range of tilt angles up to about 40 deg., and contrast of only 1 can be achieved at a tilt angle of 80 deg.
In order to improve characteristics in such diagonal viewing directions, the above-cited prior patent application has disclosed a configuration in which an optical retardation film is disposed in an MVALCD. However, the improvement achieved by the configuration disclosed in the prior patent application is insufficient, and there are demands for further improvements.